With the spread of personal computers in recent years, digital cameras capable of capturing an image easily into the personal computer have come into widespread use. Further, it has become a common practice to incorporate a digital camera into an information processing apparatus such as a mobile computer, cellular phone and PDA (Personal Digital Assistant). The widespread use of such digital cameras calls for more compact digital cameras. Thus, further downsizing of an imaging optical system is an essential technique to keep up with the trend.
To downsize the imaging optical system, the Japanese Laid-Open Patent Publication No. 2000-131610 discloses a proposal wherein a reflective optical element such as a prism or reflective mirror is arranged at some midpoint along the optical path of the imaging optical system to bend the optical path, whereby the size of the imaging optical system can be reduced without changing the length of the optical path even during zooming. To meet the requirements for higher zoom ratio in the bent imaging optical system, the Japanese Laid-Open Patent Publication No. 1997-138347 proposes a zoom lens characterized by the zoom ratio of six or more. Since the first lens group is provided with a reflective optical element, the size of the reflective optical element is increased against the demand for more compact imaging optical system.
The Japanese Laid-Open Patent Publication No. 2002-169088 and Japanese Laid-Open Patent Publication No. 2004-102089 disclose the imaging optical system wherein the reflective optical element is installed closer to the image surface than the first lens group, thereby reducing the size further.
In the optical system described in the Laid-Open Patent Publication No. 2002-169088, however, a reflective optical element is provided between the second lens group and third lens group. Since many lens are arranged on the object side of the reflective optical element, this method fails to achieve downsizing the size in the direction of the depth of the digital camera.
In the optical system described in the Japanese Laid-Open Patent Publication No. 2004-102089, a reflective optical element is arranged in the second lens group, thereby reducing the size of the digital camera along the depth. However, the thickness from the reflective optical element to the most object side surface of the optical system is not yet reduced. The target of downsizing cannot be said to have been achieved.